The present invention relates in general to an aeroengine nacelle afterbody of the type comprising a common nozzle for exhausting the mixed hot and cold gases originating respectively from the fan duct and from the aeroengine combustion chamber, the said nozzle having a longitudinal axis more or less coincident with the axis of the said engine.
The invention therefore relates to what is commonly known as an LDMF (or long duct mixed flow) nacelle afterbody.
In known afterbodies of this type, the CNA (common nozzle assembly) is either fixed, via radial arms, to a primary nozzle, itself secured to the turbine housing, or fixed to the downstream end of the thrust reverser fairing which surrounds the compressors/combustion chamber/turbine assembly. In both cases, the CNA is made of a single part, whereas, in general, the fairing in which the aforementioned assembly is included consists of two half-fairings which are, on the one hand, individually hinged to a support structure, and, on the other hand, lockable together opposite the said hinge.
The aforementioned support structure is either the engine strut if the engine is installed in a nacelle under the wing, or a pylon structure, itself mounted on the aft fuselage of the aircraft, if the engine is installed in a lateral nacelle.
Attachment of the CNA poses numerous problems, particularly given the increasingly tight safety standards in case a fan blade should break and become detached. When such an incident (known as xe2x80x9cfan blade offxe2x80x9d) occurs, the fragment of blade which has become detached, on the one hand, strikes the surrounding parts, which entails providing reinforcements capable of preventing it from being able to cut through vital parts of the aircraft and, on the other hand, leads to an imbalance with an out-of-balance effect which causes violent vibration. In this case, it is necessary to shut down the corresponding engine, with the result that the turbine windmills, something which reduces but does not completely eliminate the harmful vibrations.
The problem of attaching the CNA becomes all the greater if there is a desire to install a thrust reverser in this CNA, because of the weight added by the reverser and the particular forces to be transmitted.
According to a first aspect of the invention, provision is made, in order to reduce the effect of mounting the common nozzle (CNA) with an overhang, for this nozzle to be made in the form of two half-nozzles which are more or less symmetric with respect to a plane containing the said longitudinal axis, the said half-nozzles being, on the one hand, individually hinged via one of their longitudinal edges to the said support structure and, on the other hand, lockable together along their opposite longitudinal edges.
The CNA is furthermore attached to the downstream end of the fairing by collaboration between a part forming a joint, having a V-shaped part, provided in the downstream end of the said fairing and a complementary V-shaped groove formed in the end facing it of the CNA (an assembly known to those skilled in the art by the name of a V-blade/V-groove structure). Such a structure, illustrated for example in FIG. 4 of U.S. Pat. No. 4,998,409 is designed to transmit axial forces and withstand them. Now, if a fan blade breaks and becomes detached, the vibrations cause forces in uncontrolled directions which the V-blade/V-groove structure may have difficulty in withstanding. Admittedly, more sophisticated V-blade/V-groove structures have been developed (for examples ones using 90xc2x0 dovetail profiles to avoid any rotational movement) but attachment to the downstream end of the fairing remains a problem.
To overcome these drawbacks, according to a second aspect, the invention provides an afterbody of the aforementioned type and in which each half-fairing and each half-nozzle following on one from the other are made as a single piece.
Thus, the CNA is no longer a separate part requiring attachment to a separate afterbody, and it follows that the problems inherent with this attachment are eliminated
The union of the fairing and of the CNA into a single assembly formed of two hinged parts, aside from solving the aforementioned problem, presents numerous advantages which are as follows:
by dispensing with the means of attachment to a separate afterbody, savings are made not only in material and labour, but above all in nacelle weight; now, in aeronautics, any weight saving has an appreciable economic impact during operation;
by replacing the one-piece CNA by incorporating the CNA into the two-half structure of the fairing, the structure in question is admittedly lengthened but its bulk in terms of cross section is reduced, this making it easier to transport and to handle, the cross-sectional bulk often being more difficult to deal with than the lengthways bulk;
and, above all, by having a unitary afterbody, there is far greater freedom to choose where a thrust reverser will be mounted: for example, the reverser could be made to act only on the cold flow if the doors are installed in the afterbody upstream of the downstream edge of the turbine or alternatively could be made to act on the mixture of cold and hot flows if it is installed downstream (as seen above, such downstream mounting is difficult to envisage in the case of a separate CNA because of the additional offset weight that the doors and other auxiliary parts of the reverser contribute).
This possibility of installing the thrust reverser in such a way that it acts on the mixture of flows, while keeping a part of CNA structure downstream, in turn, affords a considerable advantage.
When the thrust reverser acts only on the cold flow, there remains, at the time of thrust reversal, a thrust in direct-jet mode, provided by the hot flow, while braking is by means of the reversal of the cold flow. The braking is therefore the result of the difference between the thrust of the hot flow and the thrust-reversal of the cold flow and, to be satisfactory, requires a high, and therefore noisy engine speed.
When the thrust reverser acts both on the cold flow and on the hot flow, the braking ability is better. It is thus possible either, for the same engine speed, to obtain more radical braking or, for the same intensity of braking, to run the engine at a lower speed, therefore with a substantial reduction in the noise.
Now, this question of noise is increasingly penalizing to airlines, in that a certain number of airports have already forbidden the use of thrust reversal beyond a certain time in the evening. It sometimes follows, in the event of a delayed take-off, that it is impossible to resort to thrust reversal, which presents an element of risk. Thrust reversal with a low, therefore acceptable noise level would allow thrust reversal to be used regardless of the time of landing and would free the airlines of this concern and of this risk.
The subject of the invention is therefore a long duct mixed flow nacelle afterbody for an aeroengine comprising an upstream zone and a downstream zone by reference to the direction in which the gases flow, which consists of two parts which are more or less symmetric with respect to a plane containing the longitudinal axis of the aeroengine, the said parts being, on the one hand, individually hinged by one of their longitudinal edges to the said support structure and, on the other hand, lockable together along their opposite longitudinal edges, each of the said parts comprising, from the outside inwards, in the upstream zone, three aerodynamic surfaces, namely a nacelle exterior surface, a cold flow duct exterior surface and a cold flow duct interior surface and, in the downstream zone, two aerodynamic surfaces, namely a nacelle exterior surface and a mixed hot and cold flow surface.